In an ideal world all our children would have an ideal upbringing that had mother, father, enough siblings for companionship; but not so many that resources would get stretched.
However, each of us has our limitations and relationships are no different. They fall apart for myriad different reasons, some of which may be our own fault, but then some again which may not!
When we are past apportioning blame it is best to make peace with a changed situation, which at that point would typically mean for one parent to become a single parent.
Admittedly this is not the most ideal situation, but it may well be the only workable one and it is important for a single parent to remember that.
Yes a parent benefits from having both parents around, but the child benefits from two happy and productive parents around.
If those parents are together but not happy and not leading productive lives, then this is going to have a far more profoundly negative effect on a child or children.
One parent who is doing his or her best is a more positive option for a child than two who have resentment and acrimony between them.
What a child also does not benefit from is for that one parent, a principal caretaker to beat themselves up over what-ifs and guilt. What is important here is moving on, and making the best of it for parent and child.